1. Field of the Invention
This invention relates to a radio communication system using frequency hopping, as well as to a method of controlling this system.
2. Description of the Related Art
Frequency hopping involves dividing a certain frequency band into a plurality of narrower bands, adopting each of these bands as a frequency channel and transmitting a signal while changing from one frequency to another during the time it takes to send one bit of data. The number of times frequency is changed during the transmission of this one bit is referred to as the number of hops, the pattern of this change in frequency is referred to as a hopping pattern, and the time required to cycle through the hopping pattern is referred to as the hopping period.
Since frequency is thus changed in successive fashion in accordance with frequency hopping, data can be demodulated and a high degree of security can be maintained by changing from one frequency to another even if some of the frequencies develop noise or are disturbed by other systems.
In order to realize a system which transmits information between information devices such as computers using such frequency hopping, a technique that relies upon a small, high-speed PLL (phase-locked loop) is required. However, since such a technique still does not exist, it is necessary to provide each device with a plurality of oscillators. The result is circuitry of large size.
Accordingly, in currently existing systems, frequencies are not changed successively during one bit of data. Rather, the method employed involves transmitting a signal while changing frequency every several dozen bits of data. This method is referred to as frequency slow hopping. In this case, the anti-noise property originally possessed by the frequency hopping technique is sacrificed but excellent security can be maintained.
When it is required that many devices in a system communicate at the same time, all of these devices cannot communicate using the same hopping pattern. Accordingly, communication is performed using a hopping pattern that differs for each communication or for every few communications. Consequently, a plurality of hopping patterns are required in one system. However, if all of these hopping patterns are stored in all of the system devices, it will be necessary to provide each device with a large memory capacity.
Further, in the case of a system having a main unit, a method available is to perform communication by having the main unit decide, whenever communication is carried out, the hopping pattern to be used for the particular device that is to communicate. In this case, however, notification of all frequencies used in the hopping pattern employed must be given, one frequency at a time. This means that considerable time is required to notify the units of the hopping pattern.
Generally, in a system having a main unit, all of the calls within the system are implemented via the main unit. In a case where a large number of communications are performed simultaneously, increasing the number of lines between the main unit and terminal devices on both sides thereof places a heavy load upon the main unit.
Further, in a case where a terminal device A and a terminal device B communicate via the main unit, two hopping patterns are required, namely a hopping pattern between the terminal device A and the main unit and a hopping pattern between the terminal device B and the main unit. The result is a large number of hopping patterns. Accordingly, if the distance between the two terminal devices is such that they are capable of communicating with each other directly, it is desired that communication between the terminal devices be implemented using a hopping pattern different from the hopping patterns between the main unit and the terminal devices.
In order to realize communication between terminal devices using a hopping pattern different from the hopping patterns between the main unit and the terminal devices when the distance between the two terminal devices is such that they are capable of communicating with each other directly, it is necessary that the frequency channels be selected in such a manner that the same frequency channels will not be used simultaneously in the hopping pattern employed in communication between the main unit and the terminal devices and in the hopping pattern employed in communication between the terminal devices. The reason for this is that using the same frequency channels at the same time results in interference between the frequency channels.